Bug 917550 - [mozprocess] ProcessHandler commandline attr broken;r=wlach

Consistently store `cmd` as a string and `args` as a list. Throw an
error when `args` are over-specified. Document the complete usage of the
`cmd` and `args` options.
Bug 932337 - [mozprofile] Downloaded add-on should get a meaningful filename. r=ahal
Bug 932337 - Add missing test add-on 'invalid.xpi'